This invention relates to a method of operating a filter press having a pressure tank communicating therewith. During filtration part of the sludge being pumped under pressure to the filter press is conveyed to another location and is stored in a tank or suitable vessel under a cushion of compressed air. During the subsequent charging of the filter press the sludge thus stored is introduced into the filter press under the pressure of the compressed air and after each filtering operation, the filter press is blown clean by injection of compressed air. The invention further relates to apparatus for carrying out this operation. The method and apparatus are essentially but not exclusively applicable to filter presses employed in the dewatering of sewage sludges.
The above described process comprises storing under pressure a certain amount of sludge in a tank under a cushion of compressed air. When the press is charged the sludge thus pressurized and stored is immediately introduced into the filter press. Charging the press is thus considerably accelerated and made more uniform as compared to charging by means of a feed pump. During the filtration that follows, the charging part of the slurry is forced by the feed pump from the press unfiltered whereupon it is withdrawn from the press and stored in the tank under pressure for the next charge. An additional advantage is thus obtained in that the sludge introduced by the pump can be uniformly distributed in the press.
After each filtering operation and previous to the next charge, while the connecting valves between the filter press and the pressure tank are closed, the filter press is opened, the filter cake released and the filter cloth cleaned, if necessary. Before the filter press is opened any suspension still remaining therein should be removed to prevent it from oozing into the filter cake before the latter is removed. This is done by the injection of compressed air into the press, the compressed air is usually injected at the end of the press opposite the feed pump, so that the remaining suspension is returned to the sludge sump of the pump. Before the air is injected, the press may be rinsed with pressurized liquid.
The pressure that is eventually reached during storage of the sludge in the pressure tank corresponds to the delivery pressure of the feed pump and ranges usually from 10 to approximately 15 bars or atmospheres. This is the pressure at which the sludge has normally been fed to the filter press in the systems heretofore known to the art. It has been found that the rapid flow of sludge at such pressures is detrimental to the sludge flocs and especially to the filter cloth in that it reduces their service life. It is therefore an object of this invention to overcome this problem by reducing the velocity of the sludge flow to the press by means of a pressure tank in a simple manner and without wasting compressed air, as far as possible.